JP2008031901A - Catalyst deterioration detection device for internal combustion engine - Google Patents

Abstract

Even if a post-catalyst sensor is deteriorated, a true degree of catalyst deterioration is accurately detected.
An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine, wherein a post-catalyst sensor for detecting an exhaust air-fuel ratio downstream of the catalyst and an output value of the post-catalyst sensor are inverted. Means for switching the target air-fuel ratio at the same time as reaching a predetermined judgment value VR, means for detecting a parameter correlated with the degree of deterioration of the post-catalyst sensor, and determining the judgment value from VR to VRx according to the detected parameter value And a means for correcting. It is possible to obtain an appropriate judgment value according to the degree of deterioration of the post-catalyst sensor, and even if the post-catalyst sensor is deteriorated, the influence of the deterioration is eliminated and the true degree of catalyst deterioration is accurately detected. Is possible.
[Selection] Figure 7

Description

  The present invention relates to a catalyst deterioration detection device that detects deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine.

Generally, in an internal combustion engine, a catalyst is disposed in an exhaust passage in order to purify exhaust gas. Such a catalyst, for example, a three-way catalyst, adsorbs and holds excess oxygen present in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes larger than the stoichiometric air-fuel ratio, that is, when the air-fuel ratio becomes lean. When the air-fuel ratio of the gas becomes smaller than the stoichiometric air-fuel ratio, that is, when the air-fuel ratio becomes rich, it has an O ₂ storage function for releasing the adsorbed oxygen. Therefore, during normal operation of the internal combustion engine, if the air-fuel mixture is alternately swung to the rich side or the lean side centering on the stoichiometric air-fuel ratio, when the air-fuel mixture becomes lean due to the O ₂ storage function of the three-way catalyst, excess NOx is reduced because oxygen is adsorbed and held by the catalyst, and when the air-fuel mixture becomes rich, oxygen adsorbed and held by the catalyst is released, so that HC and CO are oxidized, whereby NOx, HC and CO can be purified at the same time.

  Therefore, an air-fuel ratio sensor for detecting the exhaust air-fuel ratio is disposed in the exhaust passage upstream of the catalyst, and when the exhaust air-fuel ratio becomes lean, the fuel supply amount is increased, and when the exhaust air-fuel ratio becomes rich By reducing the fuel supply amount, the air-fuel ratio is alternately shifted to the rich side or the lean side around the theoretical air-fuel ratio, thereby NOx, HC and CO are simultaneously reduced.

By the way, when the three-way catalyst deteriorates, the exhaust gas purification rate decreases. There is a correlation between the degree of deterioration of the three-way catalyst and the degree of deterioration of the O ₂ storage function. Therefore, it is possible to detect that the catalyst has deteriorated by detecting that the O ₂ storage function has deteriorated.

  As an apparatus for detecting catalyst deterioration based on such a principle, for example, there is one disclosed in Patent Document 1. This apparatus determines an abnormality of the downstream catalyst among the upstream catalyst and the downstream catalyst arranged in series in the exhaust passage of the internal combustion engine. An inter-catalyst sensor that detects an inter-catalyst air-fuel ratio between the upstream catalyst and the downstream catalyst, and a post-catalyst sensor that detects a post-catalyst air-fuel ratio downstream of the downstream catalyst are provided. Active air-fuel ratio control is performed in which the pre-catalyst air-fuel ratio upstream of the upstream catalyst is switched from lean to rich or vice versa in response to switching of the output of the inter-catalyst sensor from rich to lean or vice versa. An abnormality of the downstream catalyst is determined based on the sensor output emitted from the post-catalyst sensor during the execution of the active air-fuel ratio control.

  As another conventional technique, for example, Patent Document 2 discloses a technique for determining catalyst deterioration based on the upstream O2 sensor on the upstream and downstream side of the catalyst and the output of the downstream O2 sensor and the trajectory length thereof. Patent Document 3 discloses a technique for correcting the catalyst deterioration determination threshold based on the difference between the output signals of the gas concentration sensor respectively output under the air-fuel ratio rich condition and the air-fuel ratio lean condition. Document 4 discloses a technique for correcting a deterioration determination value according to the degree of deterioration when at least one of the first and second air-fuel ratio sensors on the upstream and downstream sides of the catalyst is detected.

JP 2004-176615 A Japanese Patent Laid-Open No. 5-163989 Japanese Patent No. 2745761 JP-A-8-100635

  By the way, when executing the active air-fuel ratio control, the output value of the post-catalyst sensor provided on the downstream side of the catalyst is reversed and reaches a predetermined rich judgment value or lean judgment value, and at the same time, the target air-fuel ratio is forcibly switched. The air-fuel ratio of exhaust gas supplied to the catalyst may be switched in the same manner as the target air-fuel ratio. In this case, the target air-fuel ratio is switched at the timing when the output value of the post-catalyst sensor reaches the rich judgment value or lean judgment value. However, when the post-catalyst sensor deteriorates, its output characteristics change, and the sensor new If the same rich judgment value or lean judgment value as that at the time is used, the deterioration degree of the catalyst may not be detected accurately. That is, the true catalyst deterioration degree is detected by adding the catalyst deterioration degree corresponding to the post-catalyst sensor deterioration, and the true catalyst deterioration degree may not be detected accurately.

  Therefore, the present invention has been made in view of such circumstances, and its purpose is to catalyze deterioration of an internal combustion engine that can accurately detect the true degree of catalyst deterioration even when the post-catalyst sensor is deteriorated. It is to provide a detection device.

In order to achieve the above object, the first invention provides:
An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A post-catalyst sensor for detecting the exhaust air-fuel ratio downstream of the catalyst;
Active air-fuel ratio control means for forcibly switching the target air-fuel ratio to a predetermined lean air-fuel ratio or rich air-fuel ratio at the same time that the output value of the post-catalyst sensor is inverted and reaches a predetermined rich determination value or lean determination value;
Parameter detecting means for detecting a parameter correlated with the degree of deterioration of the post-catalyst sensor;
And determination value correction means for correcting at least one of the rich determination value and the lean determination value in accordance with a parameter value detected by the parameter detection means.

  According to the first aspect of the present invention, at least one of the rich determination value and the lean determination value is corrected according to the parameter value correlated with the deterioration degree of the post-catalyst sensor. Therefore, it is possible to obtain an appropriate rich judgment value or lean judgment value according to the degree of deterioration of the post-catalyst sensor, and even if the post-catalyst sensor deteriorates, the influence of the deterioration is eliminated, and true catalyst deterioration It is possible to accurately detect the degree.

The second invention is the first invention, wherein
The parameter is a travel distance of a vehicle on which the internal combustion engine is mounted, or a cumulative energization time in the internal combustion engine.

  The deterioration degree of the post-catalyst sensor increases as the travel distance of the vehicle on which the internal combustion engine is mounted increases and as the cumulative energization time in the internal combustion engine increases. Therefore, the values of the travel distance and the cumulative energization time are suitable as parameters that correlate with the deterioration degree of the post-catalyst sensor, and the deterioration degree of the post-catalyst sensor can be appropriately detected by using these values.

The third invention is the first or second invention, wherein
The determination value correction means performs the correction when the value of the parameter exceeds a predetermined threshold value.

  The post-catalyst sensor is almost in a deteriorated state when a parameter value such as a travel distance is within a predetermined value, but when the parameter value exceeds the predetermined value, the degree of deterioration increases. Therefore, by executing the correction when the parameter value exceeds the predetermined threshold value, it is possible to execute a preferable correction adapted to the deterioration characteristics of the post-catalyst sensor.

Further, a fourth invention is any one of the first to third inventions,
The determination value correction unit performs the correction so that the degree of correction increases as the parameter value increases.

  The degree of deterioration of the post-catalyst sensor increases as the parameter value such as the travel distance increases. Therefore, by increasing the degree of correction as the parameter value increases, it is possible to execute preferable correction adapted to the deterioration characteristics of the post-catalyst sensor.

  According to the present invention, even if the post-catalyst sensor is deteriorated, an excellent effect that the true degree of catalyst deterioration can be accurately detected is exhibited.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the configuration of the present embodiment. As shown in the figure, the internal combustion engine 1 generates power by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston 4 in the combustion chamber 3. To do. The internal combustion engine 1 is a vehicular multi-cylinder engine (only one cylinder is shown), and is a spark ignition type internal combustion engine, more specifically, a gasoline engine.

  In the cylinder head of the internal combustion engine 1, an intake valve Vi for opening and closing the intake port and an exhaust valve Ve for opening and closing the exhaust port are provided for each cylinder. Each intake valve Vi and each exhaust valve Ve are opened and closed by a camshaft (not shown). A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder. Further, an injector (fuel injection valve) 12 is disposed in the cylinder head for each cylinder so that fuel is directly injected into the combustion chamber 3. The piston 4 is configured as a so-called deep dish top surface type, and a concave portion 4a is formed on the upper surface thereof. In the internal combustion engine 1, fuel is directly injected from the injector 12 toward the concave portion 4 a of the piston 4 in a state where air is sucked into the combustion chamber 3. As a result, a layer of a mixture of fuel and air is formed (stratified) in the vicinity of the spark plug 7 and separated from the surrounding air layer, and stable stratified combustion is executed.

  The intake port of each cylinder is connected to a surge tank 8 serving as an intake air collecting chamber via a branch pipe for each cylinder. An intake pipe 13 that forms an intake manifold passage is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. An air flow meter 5 for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 in order from the upstream side. An intake passage is formed by the intake port, the surge tank 8 and the intake pipe 13.

On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 6 forming an exhaust collecting passage through a branch pipe for each cylinder, and a catalyst 11 made of a three-way catalyst having an O ₂ storage function is connected to the exhaust pipe 6. It is attached. An exhaust passage is formed by the exhaust port, the branch pipe, and the exhaust pipe 6. Pre-catalyst sensors and post-catalyst sensors 17 and 18 for detecting the exhaust air-fuel ratio are installed on the upstream side and the downstream side of the catalyst 11, respectively. The pre-catalyst sensor 17 is a so-called wide-area air-fuel ratio sensor, can continuously detect an air-fuel ratio over a relatively wide area, and outputs a current signal proportional to the air-fuel ratio. On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output voltage changes suddenly at the theoretical air-fuel ratio.

  The spark plug 7, the throttle valve 10, the injector 12, and the like described above are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 14 that detects the crank angle of the internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. An opening sensor 15, an intake pressure sensor 16 that detects intake pressure, a throttle opening sensor 19 that detects the opening of the throttle valve 10, a odometer 21 that detects the travel distance of a vehicle on which the internal combustion engine 1 is mounted, and the like These various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 is electrically connected to an ignition switch 22 for turning on and off the entire system of the internal combustion engine 1. The ECU 20 controls the ignition plug 7, the throttle valve 10, the injector 12, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel injection timing, throttle opening. Control the degree etc. The throttle opening is normally controlled to an opening corresponding to the accelerator opening.

  The catalyst 11 simultaneously purifies NOx, HC and CO when the air-fuel ratio A / F of the exhaust gas flowing into the catalyst 11 is a stoichiometric air-fuel ratio (stoichiometric) A / Fs (for example, 14.6). Correspondingly, during normal operation of the internal combustion engine, the ECU 20 controls the air / fuel ratio so that the exhaust air / fuel ratio upstream of the catalyst, that is, the pre-catalyst air / fuel ratio A / Ffr becomes the stoichiometric air / fuel ratio A / Fs. Specifically, the ECU 20 sets a target air-fuel ratio A / Ft equal to the theoretical air-fuel ratio A / Fs, and the pre-catalyst air-fuel ratio A / Ffr detected by the pre-catalyst sensor 17 matches the target air-fuel ratio A / Ft. Thus, the fuel injection amount injected from the injector 12 is controlled. As a result, the air-fuel ratio of the exhaust gas flowing into the catalyst 11 is kept in the vicinity of the theoretical air-fuel ratio, and the maximum purification performance is exhibited in the catalyst 11.

Here, the catalyst 11 will be described in more detail. As shown in FIG. 2, in the catalyst 11, a coating material 31 is coated on the surface of a carrier base material (not shown), and the coating material 31 is held in a state in which a large number of particulate catalyst components 32 are dispersedly arranged. The catalyst 11 is exposed inside. The catalyst component 32 is mainly composed of a noble metal such as Pt or Pd, and serves as an active point for reacting exhaust gas components such as NOx, HC and CO. On the other hand, the coating material 31 plays the role of a promoter that promotes the reaction at the interface between the exhaust gas and the catalyst component 32 and includes an oxygen storage component capable of absorbing and releasing oxygen according to the air-fuel ratio of the atmospheric gas. Oxygen storage component, for example made of cerium dioxide CeO _2. For example, when the atmosphere gas of the catalyst component 32 and the coating material 31 is richer than the theoretical air-fuel ratio A / Fs, oxygen stored in the oxygen storage component existing around the catalyst component 32 is released, and as a result, release Unburned components such as HC and CO are oxidized and purified by the released oxygen. Conversely, if the atmosphere gas of the catalyst component 32 and the coating material 31 is leaner than the theoretical air-fuel ratio A / Fs, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmosphere gas, and as a result, NOx is reduced. Reduced and purified.

  Even if the pre-catalyst air-fuel ratio A / Ffr slightly varies from the stoichiometric air-fuel ratio A / Fs during the normal air-fuel ratio control, the three exhaust gas components such as NOx, HC, and CO can be obtained. Can be purified simultaneously. Therefore, in normal air-fuel ratio control, it is also possible to purify exhaust gas by causing the pre-catalyst air-fuel ratio A / Ffr to oscillate slightly around the stoichiometric air-fuel ratio A / Fs and to repeatedly absorb and release oxygen.

  By the way, in the catalyst 11 in the new state, as described above, a large number of fine particulate catalyst components 32 are uniformly distributed, and the contact probability between the exhaust gas and the catalyst component 32 is kept high. However, when the catalyst 11 deteriorates, some of the catalyst components 32 are lost, and some of the catalyst components 32 are baked and solidified by exhaust heat (see broken lines in the figure). In this case, the contact probability between the exhaust gas and the catalyst component 32 is lowered, and the purification rate is lowered. In addition to this, the amount of the coating material 31 existing around the catalyst component 32, that is, the amount of the oxygen storage component decreases, and the oxygen storage capacity itself decreases.

Thus, the degree of deterioration of the catalyst 11 and the degree of decrease in the oxygen storage capacity of the catalyst 11 are in a correlation. Therefore, in this embodiment, the degree of deterioration of the catalyst 11 is detected or determined by detecting the oxygen storage capacity of the catalyst 11. Here, the oxygen storage capacity of the catalyst 11 is represented by the amount of oxygen storage capacity (OSC; O ₂ Strage Capacity, the unit is g), which is the amount of oxygen that the catalyst 11 can store.

  Hereinafter, detection of deterioration of the catalyst in the present embodiment will be described.

  In the present embodiment, active air-fuel ratio control by the ECU 20 is executed when the deterioration of the catalyst 11 is detected. Here, the active air-fuel ratio control means that the pre-catalyst air-fuel ratio A / Ffr that is the exhaust air-fuel ratio upstream of the catalyst is changed from one of the predetermined rich air-fuel ratio A / Fr and lean air-fuel ratio A / Fl to a predetermined value. This control is forcibly switched at the timing.

  Here, the deterioration detection of the catalyst 11 is executed when the internal combustion engine 1 is in a steady operation and when the catalyst 11 is in a predetermined activation temperature range. The temperature of the catalyst 11 may be detected directly, but in the present embodiment, it is estimated using a predetermined map or function based on the engine operating state. The detection of the deterioration of the catalyst 11 is performed once for each operation of the engine, and the warning device is activated when it is determined that the catalyst 11 is in a deteriorated state at least twice.

  3A and 3B, the outputs of the pre-catalyst sensor 17 and the post-catalyst sensor 18 when the active air-fuel ratio control is executed are indicated by solid lines, respectively. Further, in FIG. 3A, the target air-fuel ratio A / Ft that is an internal value of the ECU 20 is indicated by a broken line. The outputs of the pre-catalyst sensor 17 and the post-catalyst sensor 18 represent the pre-catalyst air / fuel ratio A / Ffr and the post-catalyst air / fuel ratio A / Frr, respectively.

  As shown in FIG. 3A, the target air-fuel ratio A / Ft is centered on the theoretical air-fuel ratio A / Fs as the center air-fuel ratio, and then has a predetermined amplitude (rich amplitude Ar, Ar> 0) on the rich side. ) Separated by an air-fuel ratio (rich air-fuel ratio A / Fr) and an air-fuel ratio (lean air-fuel ratio A / Fl) separated from the air-fuel ratio by a predetermined amplitude (lean amplitude Al, Al> 0) on the lean side. And alternately. Then, the pre-catalyst air-fuel ratio A / Ffr as an actual value is switched with a slight time delay with respect to the target air-fuel ratio A / Ft so as to follow the switching or vibration of the target air-fuel ratio A / Ft. Accordingly, the pre-catalyst air-fuel ratio A / Ffr is also forcibly and alternately switched between the rich air-fuel ratio A / Fr and the lean air-fuel ratio A / Fl in the same manner as the target air-fuel ratio A / Ft. From this, it will be understood that the target air-fuel ratio A / Ft and the pre-catalyst air-fuel ratio A / Ffr are equivalent except that there is a time delay.

  In the illustrated example, the rich amplitude Ar and the lean amplitude Al are equal. For example, theoretical air fuel ratio A / Fs = 14.6, rich air fuel ratio A / Fr = 14.1, lean air fuel ratio A / Fl = 15.1, rich amplitude Ar = lean amplitude Al = 0.5. Compared with the normal air-fuel ratio control, the active air-fuel ratio control has a larger amplitude of the air-fuel ratio, that is, the values of the rich amplitude Ar and the lean amplitude Al are larger.

  By the way, the timing at which the target air-fuel ratio A / Ft is switched is the timing at which the output of the post-catalyst sensor 18 is switched from rich to lean, or from lean to rich. As shown in the figure, the output voltage of the post-catalyst sensor 18 changes suddenly at the theoretical air-fuel ratio A / Fs, and the post-catalyst air-fuel ratio A / Frr is the rich air-fuel ratio smaller than the theoretical air-fuel ratio A / Fs. When the output voltage becomes equal to or higher than the rich determination value VR, and when the post-catalyst air-fuel ratio A / Frr is the lean air-fuel ratio greater than the theoretical air-fuel ratio A / Fs, the output voltage becomes lower than the lean determination value VL. Here, VR> VL, for example, VR = 0.59 (V) and VL = 0.21 (V).

  As shown in FIGS. 3A and 3B, when the output voltage of the post-catalyst sensor 18 changes from the rich value to the lean value and becomes equal to the lean determination value VL (time t1), the target sky The fuel ratio A / Ft is switched from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr. Thereafter, when the output voltage of the post-catalyst sensor 18 changes from the lean value to the rich side and becomes equal to the rich determination value VR (time t2), the target air-fuel ratio A / Ft becomes lean from the rich air-fuel ratio A / Fr. The air-fuel ratio is switched to A / Fl.

  In this way, the output value of the post-catalyst sensor 18 is inverted and reaches the rich determination value VR or the lean determination value VL. At the same time, the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / Fl or the rich air-fuel ratio A / Fr. It is forcibly switched.

  While performing this active air-fuel ratio control, the oxygen storage capacity OSC of the catalyst 11 is calculated as follows, and the deterioration of the catalyst 11 is determined.

  Referring to FIG. 3, the target air-fuel ratio A / Ft is set to the lean air-fuel ratio A / Fl before time t1, and the lean gas flows into the catalyst 11. At this time, the catalyst 11 continues to absorb oxygen, but when it fully absorbs oxygen, it can no longer absorb oxygen, and the lean gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Frr changes to the lean side, and when the output voltage of the post-catalyst sensor 18 reaches the lean determination value VL (t1), the target air-fuel ratio A / Ft becomes the rich air-fuel ratio A / Fr. Or reversed. In this way, the target air-fuel ratio A / Ft is reversed using the output of the post-catalyst sensor 18 as a trigger.

  This time, rich gas flows into the catalyst 11. At this time, the oxygen stored in the catalyst 11 continues to be released from the catalyst 11. Therefore, the exhaust gas of the theoretical air-fuel ratio A / Fs flows out to the downstream side of the catalyst 11 and the post-catalyst air-fuel ratio A / Frr does not become rich, so the output of the post-catalyst sensor 18 is not reversed. When oxygen is continuously released from the catalyst 11, all of the stored oxygen is eventually released from the catalyst 11, and at that time, no more oxygen can be released, and the rich gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Frr changes to the rich side, and when the output voltage of the post-catalyst sensor 18 reaches the rich determination value VR (t2), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / Fl. Can be switched to.

  The larger the oxygen storage capacity OSC, the longer the time during which oxygen can be absorbed or released. That is, when the catalyst is not deteriorated, the inversion cycle of the target air-fuel ratio A / Ft (for example, the time from t1 to t2) becomes longer, and the inversion cycle of the target air-fuel ratio A / Ft becomes shorter as the deterioration of the catalyst proceeds. .

  Therefore, using this fact, the oxygen storage capacity OSC is calculated as follows. As shown in FIG. 4, immediately after the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t1, the pre-catalyst air-fuel ratio A / Ffr as the actual value is slightly delayed with the rich air-fuel ratio A / Fr. Switch to Fr. Then, from the time t11 when the pre-catalyst air-fuel ratio A / Ffr reaches the stoichiometric air-fuel ratio A / Fs to the time t2 when the target air-fuel ratio A / Ft next reverses, the oxygen storage capacity for every minute time is given by the following equation (1). dC is calculated, and the oxygen storage capacity dC for each minute time is integrated from time t11 to time t2. In this way, the oxygen storage capacity OSC1, that is, the amount of released oxygen in this oxygen release cycle is calculated.

  Here, Q is the fuel injection amount, and the excess air amount can be calculated by multiplying the air-fuel ratio difference ΔA / F by the fuel injection amount Q. K is the proportion of oxygen contained in the air (about 0.23).

  Basically, the oxygen storage capacity OSC1 calculated once is used and compared with a predetermined threshold value (catalyst deterioration determination threshold value). If the oxygen storage capacity OSC1 exceeds the threshold value, For example, the deterioration of the catalyst can be determined such that the deterioration is normal and the oxygen storage capacity OSC1 is lower than the threshold value. However, in this embodiment, in order to improve the accuracy, the oxygen storage capacity (oxygen absorption amount in this case) is calculated on the lean side as well, and the calculation is repeated a plurality of times on the rich side and the lean side as necessary. The final deterioration judgment is performed by comparing the value with a threshold value.

  Specifically, as shown in FIG. 4, after the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio A / Fl at time t2, the oxygen storage capacity dC for every minute time is calculated by the previous equation (1). And the oxygen storage capacity dC for each minute time from the time t21 when the pre-catalyst air-fuel ratio A / Ffr reaches the stoichiometric air-fuel ratio A / Fs, and then when the target air-fuel ratio A / Ft reverses to the rich side Integration is performed until t3. Thus, the oxygen storage capacity OSC2, that is, the amount of absorbed oxygen in this oxygen absorption cycle is calculated. The oxygen storage capacity OSC1 of the previous cycle and the oxygen storage capacity OSC2 of the current cycle should be approximately equal. Thus, a plurality of oxygen storage capacities OSC1, OSC2,... OSCn (for example, n is 5 or more) are repeatedly calculated, and the average value OCCSav is compared with a predetermined threshold value OSCs. If the average value OCCSav exceeds the threshold value OSCs, the catalyst 11 is determined to be normal, and if the average value OCCSav is equal to or less than the threshold value OSCs, the catalyst 11 is determined to be deteriorated.

  Note that the calculation number n of the oxygen storage capacity OSC may be changed in accordance with a value that correlates with the degree of catalyst deterioration, such as the travel distance of the vehicle. For example, if it is assumed that the travel distance is relatively small and the deterioration is not significantly advanced, the value of n is set to a small value. If the travel distance is relatively large and the deterioration may be advanced to a considerable degree, the value of n is increased. And

  Here, the relationship between the oxygen storage capacity OSC and the catalyst temperature is shown in FIG. As can be seen, the oxygen storage capacity OSC tends to increase as the catalyst temperature increases. The oxygen storage capacity OSC is the maximum for a new catalyst, and decreases as the catalyst deteriorates. When the oxygen storage capacity OSC becomes equal to or less than the threshold value OSCs, the catalyst 11 is determined to be deteriorated.

  In the active air-fuel ratio control described above, the output voltage of the post-catalyst sensor 18 is reversed to the lean side and reaches the lean determination value VL (t1 in FIG. 3). At the same time, the target air-fuel ratio A / Ft is rich. At the same time that the output voltage of the post-catalyst sensor 18 is reversed to the rich side and reaches the rich determination value VR (t2 in FIG. 3), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / Fl. Can be switched. Here, the lean determination value VL and the rich determination value VR are values set in advance as appropriate values on the assumption that the post-catalyst sensor 18 is new or undegraded. On the other hand, when the post-catalyst sensor 18 deteriorates, its output characteristics change. Therefore, in this case, if the same judgment values VL and VR as when new or undegraded are used, the true deterioration degree of the catalyst may not be detected accurately.

  FIG. 6 shows the relationship between the degree of deterioration of the catalyst 11 and the post-catalyst sensor 18 and the oxygen storage capacity OSC of the catalyst 11 calculated as described above. Here, the initial state of the catalyst 11 and the post-catalyst sensor 18 is assumed to be new, and the horizontal axis indicates the degree of deterioration when both are used for the same time. As shown in the figure, the oxygen storage capacity OSC is calculated as a high value as indicated by Z in the initial state, but gradually decreases as the degree of deterioration increases. By the way, the calculated value of the oxygen storage capacity OSC is obtained by subtracting the decrease due to deterioration of the catalyst 11 itself and the decrease due to deterioration of the post-catalyst sensor 18 from the new value Z. That is, as a result of subtracting the deterioration amount of the post-catalyst sensor 18, the calculated value of the oxygen storage capacity OSC is calculated to be smaller than the value of the deterioration amount of the catalyst 11 alone. Therefore, the true catalyst deterioration degree considering only the deterioration of the catalyst 11 alone cannot be accurately detected, and the calculated oxygen storage capacity OSC value is a deterioration determination threshold value for a catalyst that has not yet sufficiently deteriorated. A misjudgment that may be determined to be deteriorated below OSCs may occur.

  This will be described in more detail with reference to FIG. FIG. 7 shows a comparison of the respective values when the post-catalyst sensor 18 is new and when it is deteriorated. When the sensor 18 is new, it is indicated by an alternate long and short dash line and when it is deteriorated by a solid line. (A) and (B) show the target air-fuel ratio A / Ft when the post-catalyst sensor 18 is new and deteriorated, respectively. (C) and (D) are when the post-catalyst sensor 18 is new and deteriorated, respectively. (E) and (F) show calculated values of the oxygen storage capacity OSC when the post-catalyst sensor 18 is new and deteriorated, respectively. It is assumed that the catalyst is the same when new and deteriorated.

  When the post-catalyst sensor 18 shown in (A), (C), and (E) is new, when the output voltage of the post-catalyst sensor 18 is reversed from the lean side to the rich side and reaches the rich determination value VR, the target is simultaneously reached. The air-fuel ratio A / Ft is switched from the rich air-fuel ratio A / Fr to the lean air-fuel ratio A / Fl. At the same time, the accumulation of the oxygen storage capacity OSC ends, and a relatively large value OSCz of the oxygen storage capacity is obtained.

  However, in the case of the deterioration of the post-catalyst sensor 18 shown in (B), (D), and (F), the responsiveness of the post-catalyst sensor 18 is faster than when it is new. This is considered to be because the catalyst component 32 made of noble metal scattered from the catalyst 11 adheres to the post-catalyst sensor 18. Therefore, when the output voltage of the post-catalyst sensor 18 reverses from the lean side to the rich side, the reverse speed increases (that is, the slope of the diagram becomes steep), and reaches the rich determination value VR earlier than when it is new. . In this case, the target air-fuel ratio A / Ft is switched from the rich air-fuel ratio A / Fr to the lean air-fuel ratio A / Fl at a timing earlier than when new, and in particular, the accumulation of the oxygen storage capacity OSC ends earlier than when new. As a result, the oxygen storage capacity value OSCzx, which is smaller than that of the new product, is calculated.

  This illustrated example is a case where the output voltage of the post-catalyst sensor 18 is inverted from the lean side to the rich side, but in the opposite case, that is, when the output voltage of the post-catalyst sensor 18 is inverted from the rich side to the lean side. The same applies to the case. Although not shown, in this case as well, the output voltage of the post-catalyst sensor 18 reaches the lean determination value VL earlier than when it is new, and the oxygen storage capacity value smaller than when new is calculated. Become. In this way, the value of the oxygen storage capacity OSC smaller than the true value is calculated due to the deterioration of the post-catalyst sensor 18, and an erroneous determination may be made that determines that the catalyst has not deteriorated sufficiently.

  Therefore, in order to accurately detect the true catalyst deterioration level even when the post-catalyst sensor 18 is deteriorated, in the present embodiment, at least the rich determination value VR and the lean determination value VL are determined according to the deterioration degree of the post-catalyst sensor 18. One (both in the present embodiment) is corrected.

  Specifically, as shown in FIG. 7, when the post-catalyst sensor 18 deteriorates, the rich determination value VR is corrected to a higher value VRx. Here, the correction amount ΔVR is ΔVR = VRx−VR. This correction amount ΔVR has a timing when the deteriorated post-catalyst sensor 18 reaches the corrected rich determination value VRx and a timing when the new post-catalyst sensor 18 reaches the reference rich determination value VR for the same catalyst 11. It is preset so as to be the same. By this correction, the timing at which the output voltage of the post-catalyst sensor 18 reaches the rich determination value is delayed, and the integration of the oxygen storage capacity can be completed at the same timing as when the post-catalyst sensor 18 is deteriorated. Therefore, it becomes possible to calculate the true oxygen storage capacity value OSCz that is the same as when the sensor is new, and it is possible to prevent erroneous determination of catalyst deterioration.

  Although not shown, on the lean side, when the post-catalyst sensor 18 deteriorates, the lean determination value is corrected to a value VLx lower than the reference value VL. The correction amount ΔVL is ΔVL = VL−VLx. Similarly, with respect to the same correction amount ΔVL, for the same catalyst 11, timing when the deteriorated post-catalyst sensor 18 reaches the corrected lean determination value VLx and timing when the new post-catalyst sensor 18 reaches the reference lean determination value VL. Are set in advance so that they are the same. With this correction, the same effect as when the rich determination value is corrected can be obtained.

  Next, correction processing of the rich determination value VR and the lean determination value VL (hereinafter also simply referred to as “determination value”) executed by the ECU 20 will be described with reference to FIG.

  First, in step S101, it is determined whether an execution condition for active air-fuel ratio control is satisfied. For example, after the ignition switch 22 is turned on, this condition is satisfied when the internal combustion engine 1 is first in a steady operation state and the estimated temperature of the catalyst 11 enters a predetermined activation temperature range. If the execution condition for the active air-fuel ratio control is not satisfied, the present process is terminated. If the execution condition for the active air-fuel ratio control is satisfied, the process proceeds to step S102.

  In step S102, whether or not the degree of deterioration of the post-catalyst sensor 18 exceeds a predetermined level, specifically, a parameter correlated with the degree of deterioration of the post-catalyst sensor 18 (referred to as a deterioration parameter) has a predetermined threshold value. It is determined whether it has been exceeded. This type of deterioration parameter can be, for example, the travel distance Lv of the vehicle on which the internal combustion engine 1 is mounted. This is because the post-catalyst sensor 18 deteriorates as the travel distance Lv increases. In this case, the travel distance Lv from the time of attachment of the post-catalyst sensor 18 (including when it is first attached and when it is attached by replacement) is determined by using the measured value of the odometer 21 to the ECU 20. Is measured by Then, it is determined whether or not the travel distance Lv exceeds a predetermined threshold value Lvs. If the travel distance Lv does not exceed the threshold value Lvs, it is assumed that the post-catalyst sensor 18 has not deteriorated, and this processing is terminated. If the travel distance Lv exceeds the threshold value Lvs, the post-catalyst sensor 18 It is considered that the sensor 18 has deteriorated, and the process proceeds to step S103. In this case, the odometer 21 and the ECU 20 constitute parameter detection means for detecting a deterioration parameter.

  Alternatively, the deterioration parameter may be a cumulative energization time in the internal combustion engine 1. In this case, from the point of time when the post-catalyst sensor 18 is attached, the cumulative time during which the ignition switch 22 is turned on is measured by the ECU 20 using a built-in timer, and the cumulative energization time Ton thus obtained is a predetermined threshold value. It is determined whether or not Tons has been exceeded. If the cumulative energization time Ton does not exceed the threshold value Tons, the post-catalyst sensor 18 is regarded as not deteriorated, and this processing is terminated. If the cumulative energization time Ton exceeds the threshold value Tons, Considering that the post-catalyst sensor 18 has deteriorated, the process proceeds to step S103.

  In step S103, the determination value of the post-catalyst sensor 18 is corrected. This correction may be performed, for example, by adding or subtracting a predetermined constant value to the reference determination values VR and VL. However, in this embodiment, deterioration is performed using a predetermined correction map as shown in FIGS. A correction value corresponding to the parameter is extracted, and correction is performed using this correction value. Hereinafter, although the case where the deterioration parameter is the travel distance Lv will be described as an example, the same correction method is possible even when the deterioration parameter is the cumulative energization time Ton. The correction map can be replaced with a functional expression.

  In the correction map shown in FIG. 9, a correction amount ΔV that increases as the travel distance Lv increases is set in advance through experiments or the like and stored in the ECU 20. Here, although the correction amount ΔV is input even in the region below the threshold value Lvs, the value of the correction amount ΔV is substantially zero or a value in the vicinity thereof and is not used for correction. Only the correction amount ΔV larger than the zero by a predetermined value or more in the region exceeding the threshold value Lvs is used for correction. In this region, the correction amount ΔV increases as the travel distance Lv increases.

  For example, when the correction for raising the rich determination value as shown in FIG. 7 is performed, the correction amount ΔV corresponding to the current travel distance Lv, that is, the rich correction amount ΔVR is extracted from the correction map of FIG. The rich correction amount ΔVR is added to the reference rich determination value VR, and a corrected rich determination value VRx (= VR + ΔVR) is calculated. The correction of the rich determination value is completed by replacing the corrected rich determination value VRx with the reference rich determination value VR.

  When the next catalyst deterioration detection or active air-fuel ratio control is executed, the current corrected rich determination value VRx is replaced with the corrected rich determination value VRx ′ calculated next time by the same method. In this way, each time the catalyst deterioration detection or active air-fuel ratio control is executed, the rich determination value is sequentially corrected and updated according to the deterioration degree of the post-catalyst sensor 18 at that time.

  On the other hand, when the correction for reducing the lean determination value is performed, the correction amount ΔV corresponding to the current travel distance Lv, that is, the lean correction amount ΔVL, is extracted from the correction map of FIG. 9, and this lean correction amount ΔVL is the reference rich determination value. A lean determination value VLx (= VL−ΔVL) after subtraction from VL is calculated. Then, the corrected lean determination value VLx is replaced with the reference lean determination value VL, and the correction of the lean determination value is completed. The lean determination value is corrected and updated sequentially every time the catalyst deterioration detection or active air-fuel ratio control is executed.

  Next, the case where the correction map shown in FIG. 10 is used will be described. In this correction map, a correction coefficient B that increases as the travel distance Lv increases is set in advance through experiments and stored in the ECU 20. Here, as with the correction amount ΔV, the correction coefficient B is also input in the region below the threshold value Lvs, but the value of the correction coefficient B is substantially 1 or a value near it, Not used for. Only the correction coefficient B larger than the threshold value Lvs by a predetermined value or more in the region exceeding the threshold value Lvs is used for correction. In this region, the correction coefficient B increases as the travel distance Lv increases.

  For example, when the correction for raising the rich determination value as shown in FIG. 7 is performed, the correction coefficient B corresponding to the current travel distance Lv, that is, the rich correction coefficient Br is extracted from the correction map of FIG. Then, the correction coefficient Br is multiplied by the reference rich determination value VR to calculate a corrected rich determination value VRx (= VR × Br). The correction of the rich determination value is completed by replacing the corrected rich determination value VRx with the reference rich determination value VR. The rich determination value is corrected and updated sequentially every time the catalyst deterioration detection or active air-fuel ratio control is executed.

  On the other hand, when the correction for lowering the lean determination value is performed, the correction coefficient B corresponding to the current travel distance Lv, that is, the lean correction coefficient Bl is extracted from the correction map of FIG. The value (2-Bl) obtained by subtracting the value of the correction coefficient Bl is multiplied to calculate a corrected lean determination value VLx (= VL × (2-Bl). Then, the corrected lean determination value VLx. Is replaced with the reference lean determination value VL, and the correction of the lean determination value is completed, and the rich determination value is corrected and updated sequentially each time the catalyst deterioration is detected or the active air-fuel ratio control is executed.

  Thus, the present process ends. In the correction process of step S103, the corrected rich determination value VRx and lean determination value VLx become far away from the reference rich determination value VR and lean determination value VL as the travel distance Lv increases. The degree is increased. Accordingly, it is possible to execute a preferable correction adapted to the deterioration characteristic of the post-catalyst sensor 18 such that the response becomes faster as the degree of deterioration of the post-catalyst sensor 18 increases.

  As described above, according to this embodiment, it is possible to obtain an appropriate rich / lean determination value according to the degree of deterioration of the post-catalyst sensor 18, and when the post-catalyst sensor 18 deteriorates, the true value is obtained. It is possible to prevent a small value of the oxygen storage capacity OSC from being calculated. Then, the influence of the deterioration of the post-catalyst sensor 18 is eliminated, and the deterioration degree of only the true catalyst can be accurately detected. As a result, it is possible to prevent an erroneous determination to determine that a catalyst that has not deteriorated is deteriorated.

  In the above-described embodiment, the ECU 20 and the injector 12 constitute an active air-fuel ratio control means, and the ECU 20 constitutes a determination value correction means.

  The preferred embodiment of the present invention has been described in detail above, but various other embodiments of the present invention are conceivable. For example, although the above-described internal combustion engine is a direct injection type, the present invention is also applicable to an intake port (intake passage) injection type or a dual injection type internal combustion engine having both injection types. In the embodiment, both the rich determination value and the lean determination value are corrected. However, an embodiment in which only one of the rich determination value and the lean determination value is corrected is also possible. Various parameters other than the travel distance and the accumulated energization time can be adopted as the deterioration parameter.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

It is the schematic which shows the structure of this embodiment. It is a schematic sectional drawing which shows the structure of a catalyst. 3 is a time chart for explaining the basics of active air-fuel ratio control. FIG. 4 is a time chart similar to FIG. 3 for illustrating a method for calculating the oxygen storage capacity. It is a graph which shows the relationship between oxygen storage capacity and catalyst temperature, and is a figure for demonstrating the catalyst degradation determination method. It is a graph which shows the relationship between the deterioration degree of a catalyst and a post-catalyst sensor, and the oxygen storage capacity calculation value of a catalyst. It is a time chart which shows the comparison of each value when a post-catalyst sensor is new and when it is deteriorated. It is a flowchart of a judgment value correction process. An example of a correction map is shown. Another example of the correction map is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 3 Combustion chamber 5 Air flow meter 6 Exhaust pipe 7 Spark plug 10 Throttle valve 11 Catalyst 12 Injector 15 Accelerator opening sensor 17 Pre-catalyst sensor 18 Post-catalyst sensor 19 Throttle opening sensor 20 Electronic control unit (ECU)
21 Odometer 22 Ignition switch A / F Air-fuel ratio A / Ffr Pre-catalyst air-fuel ratio A / Frr Post-catalyst air-fuel ratio A / Ft Target air-fuel ratio A / Fs Theoretical air-fuel ratio A / Fr Rich air-fuel ratio A / Fl Lean air-fuel ratio OSC catalyst oxygen storage capacity OSCs deterioration determination threshold value VR rich determination value VL lean determination value VRx corrected rich determination value VLx corrected lean determination value ΔV correction amount B correction coefficient Lv travel distance Lvs travel distance threshold Ton Cumulative energization time Tons Cumulative energization time threshold

Claims (4)

An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A post-catalyst sensor for detecting the exhaust air-fuel ratio downstream of the catalyst;
Active air-fuel ratio control means for forcibly switching the target air-fuel ratio to a predetermined lean air-fuel ratio or rich air-fuel ratio at the same time that the output value of the post-catalyst sensor is inverted and reaches a predetermined rich determination value or lean determination value;
Parameter detecting means for detecting a parameter correlated with the degree of deterioration of the post-catalyst sensor;
An apparatus for detecting catalyst deterioration of an internal combustion engine, comprising: determination value correction means for correcting at least one of the rich determination value and the lean determination value in accordance with a parameter value detected by the parameter detection means.   2. The catalyst deterioration detection apparatus for an internal combustion engine according to claim 1, wherein the parameter is a travel distance of a vehicle on which the internal combustion engine is mounted, or a cumulative energization time in the internal combustion engine.   3. The catalyst deterioration detecting apparatus for an internal combustion engine according to claim 1, wherein the determination value correcting means executes the correction when the value of the parameter exceeds a predetermined threshold value. 4. The catalyst deterioration detection apparatus for an internal combustion engine according to claim 1, wherein the determination value correction means executes the correction so that the degree of correction increases as the parameter value increases.

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